Well, financially it's hard to say. Of course, food and vaccination costs will double, approximately. But one dog with medical problems can cost FAR more over its lifetime than two ... or even ten ... healthy dogs will. It's really impossible to say.

As for behavior - there are two sides to that coin.

On one hand, an "only dog" behaves much more like a person than a dog from a "multi-dog" household. If human beings are the only-dog's pack and role models, the dog emulates the behavior and social customs of that human pack. I have a few friends who swear they will NEVER own more than one dog at a time, because they like their dog to "act like a person, and not a dog." And, if you have enough time to devote to an only-dog, that one dog/one person bond can be great for both.

On the other hand, an "only dog" can be extremely needy, lonely and even a bit neurotic. My first dog, Lacie, was like this. She was fine for the first six months of her life, because I was finishing college and not working. She was with me constantly. Then, when I got a real job, she was suddenly alone ten hours per day. She ate my house, and my neighbors were ready to kill her. So, I got her a pet ... Sadie the basset hound. Yes, Lacie acted more "doggy" than she did before, but the other problems were solved. They were both very happy until Sadie died, twelve years later.